The invention relates to a system for removing foreign matter adhering to the photosensitive body or the developer carrying body of an image forming apparatus, in which foreign matter such as an aggregate of particles adhering to the surface of the photosensitive body of the image forming apparatus is separated from toner particles forming a developed image before a transfer position or adsorbed by an electrode, or a carrier on the surface of the photosensitive body or foreign matter on the surface of the developer carrying body is similarly removed.
Foreign matter such as an aggregate of toner particles which is larger in size than a toner particle has often been found adhering to the surface of a photosensitive body while mixed with normal toner particles that form a developed image in conventional image forming apparatuses. If such large foreign matter is present on the surface of the photosensitive body, such large foreign matter acts so as to prevent a transfer sheet and the photosensitive body from coming in uniform contact with each other during transfer of a developed image.
For this reason, defective transfer occurs at and around the portion where the foreign matter is present, causing a so-called "transfer deletions" in an image transferred on the transfer sheet. In a full-color copying machine or the like, in particular, since four colors, which are black, magenta, cyan, and yellow, are transferred while overlapped one upon the other, even if the number of foreign substances adhering to the surface of the photosensitive body is small for a single color, four-folded transfer deletions are finally produced on the transfer sheet, thus largely affecting the image quality.
Moreover, in the color copying machine, only the slightest transfer deletions leads to noticeable color shading in the overlap-transferred image on the sheet, thus affecting the image quality to a greater extent than in the monochromatic copying machine.
Production of aggregates of toner particles within a developing unit or infiltration of foreign matter from outside to the developing unit, which are causes of transfer deletions, are hard to obviate completely and surely. To overcome this problem, a method of removing foreign matter such as aggregates of toner particles adhering to the surface of the photosensitive body before the transfer position has often been adopted conventionally.
Conventional methods of removing foreign matter are: e.g.,
(1) A method of removing foreign matter by suction while providing a duct in proximity to the surface of the photosensitive body and sucking the foreign matter utilizing sucking forces provided by the air flowing in the duct (Japanese Patent Unexamined Publication No. 195363/1990);
(2) A method of adsorbing foreign matter on an electrode utilizing coulomb forces while providing the electrode in proximity to the surface of the photosensitive body and applying a dc component to the electrode (Japanese Patent Unexamined Publication No. 8681/1988).
However, the conventional methods (1) and (2) of removing the foreign matter possess the following problems.
That is, method (1) not only entails comparatively large-scale equipment but also high cost. Further, in terms of performance, large foreign matter such as aggregates of toner particles whose sizes are up to about 500 .mu.m can be sucked without accompanying normal toner particles that form a developed image on the surface of the photosensitive body, but if small foreign matter whose size is smaller than 500 .mu.m are to be sucked together, stronger sucking forces are required, and this causes normal toner particles forming the developed image to be unpreferably sucked or disturbed. As a result, the image quality may be affected thereby.
According to various test results, even foreign matter whose size is about 100 .mu.m affects the image quality. Thus, there is a limit in the performance of the method attempting to separate the foreign matter from the surface of the photosensitive body by the sucking forces of air.
Further, method (2) addresses the same problems as method (1). That is, if the electric field intensity is increased to adsorb the foreign matter whose size is about 100 .mu.m onto the electrode by coulomb forces, the normal toner particles that form a developed image are adsorbed along with the foreign matter and disturbed, thus affecting the image quality in some cases.